Manufacturing yield in the fabrication of integrated circuits and micromachine devices can be increased by first inspecting one or both major surfaces of a semiconductor wafer from which the circuits or devices are fabricated. Surface inspection of semiconductor wafers and the like may also take place at various subsequent stages of a fabrication process. A typical surface inspection apparatus includes an optical system that causes a light beam to scan across a surface of the substrate of interest. Information regarding the surface is determined by processing data related to the reflection or scattering of light from the surface.
U.S. Pat. No. 5,076,692 to Neukermans et al. describes an optical system having a laser source and optical elements for scanning a semiconductor wafer. A deflection mirror pivots on a spindle to cause a beam from the laser source to strike a lens in a scanning fashion. The scan from the deflection mirror to the lens is one in which the scan is a continuous change in the angle of the beam to the lens. On the other hand, the lens causes the beam to scan "telecentrically," i.e. the beam remains parallel to an arbitrary reference plane as the scanning progresses. Neukermans et al. describes the telescanning beam as having an angle of incidence of approximately seventy to eight-five degrees with respect to the normal of the surface of interest.
Telecentricity is designed to eliminate the variation in angle of incidence of the laser beam as the beam scans from one edge to the opposite edge of the surface of interest. Telecentric scanning is also described in U.S. Pat. No. 5,168,386 to Galbraith. The Galbraith patent describes use of an off-axis concave spherical mirror to achieve a telecentric scan, rather than a lens as taught by Neukermans et al. The spherical mirror, in combination with other correcting optics, can be used to achieve a uniformly focused circular beam having a desired diameter upon intersection with the surface of interest. Galbraith teaches that a typical focused spot size of 20 .mu.m can be maintained in a flat field scan over the length of a standard semiconductor wafer, i.e. 200 mm.
As previously noted, the angle of incidence of the circular beam is taught by Neukermans et al. as being within the range of seventy to eight-five degrees with respect to the normal of the surface of interest. An oblique angle of incidence is common in the art. Galbraith teaches that the scan should be substantially "flat field," i.e. imaging in a plane coinciding with the surface of interest.
A difficulty with the prior art techniques of inspecting a surface is that the techniques are susceptible to the effects of optical aberrations which tend to distort the beam. Galbraith teaches that there are difficulties in minimizing or eliminating astigmatism and coma. Particularly where the angle of incidence is shallow relative to the surface of interest, astigmatism may be an issue in the attempt to optimize the inspection process. Regardless of the angle of incidence, there is an issue of distortion introduced because the spherical mirror of Galbraith is off-axis. The beam from the off-axis spherical mirror will be somewhat distorted at the opposed extremes of the scan, in the absence of a correction mechanism.
Potentially, reliability is further reduced by use of a parabolic cylinder mirror to form a telecentrically scanning beam across a surface, as taught in U.S. Pat. No. 5,125,741 to Okada et al. The parabolic cylinder mirror of Okada et al. is aligned so that the path of an incoming beam follows the curvature of the mirror. The curvature creates the desired telecentric scan, but also focuses the beam in the direction of the scan. Consequently, the beam that strikes the surface of interest is strongly astigmatic, with a long dimension in the direction perpendicular to the direction of scan. Because the elliptical beam strikes the surface at a shallow angle of incidence, the beam is further elongated. The Okada et al. patent was designed for use in industrial fields, such as automobiles and appliances. While the Okada et al. apparatus works well in the fields for which it was designed, the apparatus is likely unusable in applications such as detecting defects and/or particles that are measured in microns or submicrons.
An object of the present invention is to provide an apparatus and method for telecentrically scanning a surface of interest in a manner to improve the inspection characteristics of an inspection instrument, such as an instrument for quality control measurements in the manufacture of semiconductor substrates.